This invention relates generally to the area of devices having a liquid interfaced with a solid component. In particular, the invention is concerned with optical systems and liquids for refractive index-matching. In one aspect the invention concerns methods for preparing a liquid composition for use in refractive index-matching.
Traditionally, signal exchanges within telecommunications networks and data communications networks have been accomplished by transmitting electrical signals via electrically conductive lines. However, an alternative medium of data exchange is the transmission of optical signals through optical fibers. Since a light beam can be easily generated and modulated so as to carry much more information than is possible with an electrical signal, light beams have a distinct advantage in communicating information between two or more locations. High quality, low loss optical fibers are now readily available so that optical fibers are now being used widely to define optical paths for such light beams. Fiber optical switches may be used to control routing of optical beams along alternative fiber optical paths. Equipment for efficiently generating and transmitting the optical signals has been designed and implemented, and optical switches have been designed for use in telecommunications and data communications networks.
There are many situations in optical systems where a liquid whose refractive index is approximately equal, or equal, to that of a solid component of the optical system, is useful such as, for example, optical switches. Most refractive index data is available for the wavelength 586.26 nanometers (nm), also known as the sodium D line. For a smaller number of materials, information is also available at a limited number of other visible wavelengths such as 706.52 nm, 667.81 nm, 656.28 nm, 546.07 nm, 501.57 nm, 486.13 nm and 435.83 nm. However, there is very little data on the refractive index of liquids in the wavelength ranges relevant to the communications industry, which are typically 1270 to 1350 nm and 1470 to 1620 nm. Techniques of estimation must be used for most compounds, and several commonly used approximations (e.g., Sellmeier equation) are not accurate due to the combined effects of ultraviolet (UV) and infra-red (IR) absorption bands. It is, therefore, not a simple matter of looking up the refractive index of a potential liquid. Experimental determination is also difficult requiring special apparatus.
Preferably, the liquid for refractive index-matching does not have atoms of chlorine, bromine or iodine as certain compounds with these elements are known to deplete the ozone layer of the earth. The liquid preferably does not contain toxic elements such as, for example, lead, tin, mercury, or other heavy metals. However, it is difficult to find compounds that have a low boiling point and sufficiently high refractive index for many applications without resorting to materials that contain chlorine, bromine, iodine, heavy metals, sulfur, selenium, tellurium or arsenic. Without these special refractive index enhancers, the refractive index largely depends on the number of atoms per unit volume. This depends on the number of atoms per molecule and the intermolecular forces that draw the molecules towards each other. However, using materials with increasing numbers of atoms per molecule and/or stronger intermolecular forces invariably increases the boiling point or lowers the vapor pressure.
There remains a need for refractive index matching liquids that have a low boiling point and sufficiently high refractive index for applications in optical systems and other areas where the refractive index of a liquid must match or approximate that of a solid component with which it is interfaced. The liquid should not comprise a material that contains chlorine, bromine, iodine, heavy metals, sulfur (in most cases), selenium, tellurium or arsenic.